1. Field of the Disclosure
This disclosure relates in general to an extractive distillation process for purifying cis-1,1,1,4,4,4-hexafluoro-2-butene.
2. Description of the Related Art
The fluorocarbon industry has been working for the past few decades to find replacement refrigerants and blowing agents for the ozone depleting chlorofluorocarbons (CFC's) and hydrochlorofluorocarbons (HCFC's) being phased out as a result of the Montreal Protocol. The solution for many applications has been the commercialization of hydrofluorocarbon (HFC) compounds for use as refrigerants, solvents, fire extinguishing agents, blowing agents and propellants. These new compounds, such as HFC refrigerants, HFC-134a and HFC-125 being the most widely used at this time, have zero ozone depletion potential and thus are not affected by the current regulatory phase-out as a result of the Montreal Protocol.
In addition to ozone depleting concerns, global warming is another environmental concern in many of these applications. Thus, there is a need for compositions that meet both low ozone depletion standards as well as having low global warming potentials. Certain hydrofluoroolefins are believed to meet both goals. Thus there is a need for manufacturing processes that provide halogenated hydrocarbons and fluoroolefins that contain no chlorine and also have a low global warming potential. The production of hydrofluoroolefins (i.e., unsaturated compounds containing only carbon, hydrogen and fluorine), has been the subject of recent interest to provide environmentally desirable products for use as effective replacements for the existing halogenated compounds.
Purification is an important step in manufacturing these compounds. Conventional distillation is typically used to separate desired products from impurities; however, conventional distillation becomes ineffective when the desired compound forms an azeotrope with or has a boiling point close to that of one or more of the impurities. For example, manufacturing 1,1,1,4,4,4-hexafluoro-2-butene (HFO-1336mzz) from CF3CCl═CClCF3 (CFC-1316mxx) can result in the formation of contaminants, such as 1,1,1,4,4,4-hexafluoro-2-chloro-2-butene (HCFC-1326mxz) and 1,1,1,2,4,4,4-heptafluoro-3-chloro-2-butene (CFC-1317mx). Cis-HFO-1336mzz and trans-HCFC-1326mxz form an azeotrope, making their complete separation by conventional distillation impossible. Cis-HFO-1336mzz forms azeotrope-like compositions with cis-HCFC-1326mxz as well, complicating removal of cis-1326mxz from cis-HFO-1336mzz as well. Cis-HFO-1336mzz also forms an azeotrope with CFC-1317mx, making their complete separation by conventional distillation impossible.
Thus, there is a need to develop other purification processes for the production of hydrofluoroolefins.